Digital input system



Nov. 17,. 1964' c. H- PROPSTER', JR 3,157,356

DIGITAL INPUT SYSTEM Filed June 13, 1961 IN V EN TOR. a. mapsrm, J12.

United States Patent Charles General This invention relates to a digital input system which utilizes a selecting circuit to transfer groups or" digital signals from a number of sources to a common utiliza tion system and more particularly to a selecting circuit which utilizes a signal translating circuit having a plurality of input terrn ls and a common output terminal for transferring a digital from a selected one of a plurality of sources to a common utilization device.

in a variety of industrial process control systems, the operating condition of many devices must often be known to determine, for example, whether they are operating in or out of limits. For that purpose appropriate digital sen ors are provided for respective devices w-. ch produce digital signals that corr spend to their operating condition. For instance, monitoring a given device for proper operation, a digital sensor may be provided w uln produces a C--volt s nal while the device is oper .ng properly within prescribed limits and a 24-volt signal only when it is operating out of limits. Sensors of that nature are genetically referred to hereinaf as signal sources because their true nature is not a lir". ing factor of the present invention.

Systems which have been employed to selectively transfer digital signals in the past have included either a stepping switch or a relay matrix to scan one digital signal source at a time. If a relatively large number of signal sources are to be scanned, such as "it-Q or more, the time required to sequentially scan them all may be too long for the purpose for which associated devices are being monitored.

in applications where rapid transfer of a large number of digital signals is required, a system may be provided for transferring groups of digital signals in parallel, particularly those digital signals associated with critical devices which must be monitored more closely. In such a system, the digital signals from a given group of sources transferred in parallel to an electronic data processor for analysis are stored therein by a flip-flop register, or similar storage device, until they are analyzed.

in order to selectively transf r groups of digital signals to a common utilization system from a plurality of sources arranged in groups, a common transfer network is provided. In the past, asymmetrical current devices, such as diodes or triodes, have been employed as buffers in such a network to eliminate sneak circuits and thereby isolate the current paths of the various digital signals. Since one asymmetrical current device must be provided to isolate the current path of each digital signal, in an expanded system having n digital signals to be transferred in groups, either :1 asymmetrical current devices must be employed or a separate transfer circuit provided for each digital signal in a network having an n-point scanner.

An object of this invention is to provide an economical way of isolating current paths coupled to a common sigrial-translating circuit such that when one digital signal is translated, it does not interact with sources of other digital signals.

Another object is to provide an economical transfer network for selectively translating digital signals in groups to a common utilization system.

These and other objects are achieved in a novel transfer network employing a plurality of current detectors, each having a low input impedance, to translate a selected group of digital signals to output terminals, each current etector being coupled to a lurality of digital current switches by isolat ng resistors, each resistor having impedance value v "ch is large relative to the low input impedance of the nt detector. in a specific embodiment, each dig. at current switch comprises contacts of a relay energized by a sig l from an associated digital sig a source such that the ital signal to be translated by the current detector is manirestcd by the open or closed condition of the relay contacts. However, it should be noted that a given digital switch may have the open or closed condition of contacts controlled mechanically by its associated device. One contact of each digital current switc 11 a group is connected to a bus bar or como... conductor. As many bus bars are protnere are distinct groups 0 ll signals to be ed. A group sel cting circuit or scanolllg a relay-type transfer tree is employed to y apply a voltage to the bus associated with the group -f digital signals selected a random for tran fer.

objects and advantages wiil become arent from following description with reference to drawings wherein the sole figure is a schematic diagram of a system for selectively transferring digital signals to a comm n util ;lon 1 inout terminals .al sources to be coupled to epresentative ch as a relay digital signal relay sv' tree having four control tively tip- 4 through the relay transfer tree when the group of digital signals a led to the relays Kl and K2 is to be selectively transferred. and are provided for select groups or" dig trated are: re and relays relays K 51 and i relays Kid-l and K142.

Each group of digital current 5 es may be expanded to accommodate as many digit" ials as necessary. For instance, each group of switches may include twenty relays. e eight bus bars are illustrated, eight groups of twenty -gital signals may be selectively transferred in order to monitor as many as devices. If a greater number of groups are to be selectively transferred, the number of bus bars may be increased by adding relays to the transfer tree comprising relays KHZ, K213 and i 4. F r instance, the addition of one more relay having eight transfer contacts increases the number of bus oars to sixteen and thereby doubles the number of groups of digital signals which may be selectively transferred.

It should be noted that although a relay transfer tree has been illustrated for selectively connecting a voltage signal to one of a plurality of bus bars, other means may be employed, such as a relay matrix or a stepping switch having as many contacts as there are bus bars.

When all of the relays K212 to K21 are de-energized, the first group of digital signals associated with the bus bar 31 is selected for transfer. However, the digital signals manifested by the open or closed condition of the transfer contacts and KZa are not translated to the output terminals 2t: and 21 until a relay K211 is enerenemas is energized by a control signal at the input teJninal ill, a negative voltage signal of approximately 125 volts is coupled to the transfer tree from a voltage divider comprising a resistor 39 having a low resistance of about twenty OhIllS connected in series with a resistor 4 3 having a high resistance of about 5K ohms.

In operation, the control signals for selectively transferring a given group of digital signals is applied to the input terminals 12 to before a control signal is applied to the input terminal ll, and the control signal applied to the input terminal ll is removed before the transfer tree is actuated to select a didercnt group of digital signals for transfer, in order to eliminate the noise which would otherwise be generated while the transfer tree is being actuated. The proper sequencing of the control signals applied to the input terminals Ill to 14 may be the outprovided by the data processing system to which put terminals 2% and 21 are connected.

For convenience only a two-p it distributor has been illustrated to selectively transfer ..Je groups of digital signals, each group comprising two relays their input terminals coupled to associated digital signal sources (not shown). t fhen the associated signal sources are operating in a given condition, the relays are in a predetermined state. For ex mple, the relays Ki t-l and K142 may be associ ted with devices having threshold detectors which provide 24-volt signals for actuating the relays when t eir operation exceeds a given value with respect to a particular characteristic.

As a specific example to clarify the operation of the digital current switch relays K141 and K142, assume that the devices with which they are associated are generators and that the characteristic to be monitored is the revolu tions per minute (rpm) of each. The t"- eshold detectors coupling the generators to the digital relay switches may comprise a tachometer, a storage counter of the staircase-type driven by the tachometer as an rpm-meter, and a voltage comparator for comparing the voltage output of tne storage counter with a reference voltage corresponding to a predetermined For the specific example, the voltage comparator is biased to provide a 24-volt signal when the voltage output signal of the storage counter exceeds the reference voltage in order to energize the associated one of the digital-currentswitch relays K143i and K142 when the corresponding generator exceeds the predetermined rpm. A suitable storage counter and voltage comparator are shown at pages 352 and 467, respectively, in a book by Jacob Millman and Herbert Taub titled Pulse and Digital Circuits, published by McGra -l-lill Book Company (1956).

The contact or" a given digital-current-switch relay is connected to its associat d current detector by a coupling resistor. For instance, the contact of the digital switch K141 is coupled to the base electrode of a transistor Q by a resistor 43., and the contact of the digital switch K142 is coupled to the base electrode of a transistor Q by a coupling resistor 42.

The transistors Q and Q are connected in a commonemitter amplifier configuration and function as current detectors having low input impedance. In operation, a current detector, such as the current detector including the transistor Q is designed to provide a O-volt signal at its output terminal when an open circuit is present in each of its input circuits and a +6-volt signal when a closed circuit is present in one of its input ci cuits.

For instance, assume that control signals are applied to the input terminals 11 to 14- to selectively transfer the last group of digital signals from the digital-currentswitch relays K141 and K142, and that the relay K1411 is de-energized as shown so that an open circuit exists between its transfer contact Kldla and the coupling resistor 41. Under those conditions, the base electrode of the transistor Q is slightly more positive than its emitter electrode connected to a +6-volt source due to current from a source of +12 volts through a resistor and a diode 1),, thereby holding the transistor Q cut off. While the transistor Q is cut off, the output terminal 2i? is clam ed at approximately 0 volts due to current from ground through a diode D and a resistor 4 to a source of +18 volts. If the relay K141 were energized, the base of the transistor Q would be connected to the source of approximately -l25 volts by the transfer contacts of the relays L212 to K214, the transfer contact Kl lla and the coupling resistor 41, thereby reverse biasing the diode D and driving the base electrode of the transistor Q below +6 volts. When the emitter of the transistor Q, is below +6 volts, the transistor conducts and reverse biases the diode D to provide an output signal of approximately +6 volts.

The advantage of utilizing a current detector having a low input impedance in accordance with the present invention is that coupling resistors, such as the resistors 41 and may be used for isolation instead of more expensive diodes or other asymmetrical current devices. The low input impedance of the transistor Q provides a D.-C. short circuit through the +6-volt to eliminate sneak circuits to the base of the transistor Q A capacitor 45 is provided as an A.-Q. short circuit to shunt transient signals produced by the digital-current-switch contacts as they are actuated in order to protect transistor Q As an example of the advantage of using a current detector having a low input impedance, consider the problem of transferring the first group of digital signals when the relay K1 is d e-energized and the relays K2, K141 and K14 are energized. Under those circumstances, when a O-volt signal is applied to the control terminals l2, l3 and 14, and a 24-volt signal is applied to the control terminal lit, the relay K211 is energized and a negative signal of approximately volts is applied through the bus bar 31, the closed transfer contact KZa, and a coupling resistor .6 to the base of the transistor Q Because the transfer contact Kla is open, a negative voltage signal is not applied to the base of the transistor Q through a coupling resistor 47 so that a O-volt signal is produced at the output terminal 2t) while a +6-volt signal is produced at the output terminal 21.

The problem of eliminat ng sneak circuits arises from the fact that the other digital switches may be closed as in the foregoing assumed problem. For instance, the negative voltage signal applied to the base of the transistor Q may be transmitted through a sneak circuit comprising the coupling resistor 42, transfer contact K1420, transfer contact 141a and the coupling resistor 41 to the base of the transistor Q to cause it to produce a +6- volt signal at the output terminal 29. However, due to the low input impedance of the transistor Q the negative voltage signal applied to the base of the transistor Q through that sneak circuit path is insuficient to overcome its cut-off bias. Thus, the provision of a low impedance current detector obviates the asymmetrical current devices employed in the past to isolate current sources coupled to a common utilization device.

It should be noted that the low impedance current detectors comprising the common-emitter connected transistor Q and Q are only illustrative and that the other types of current detectors having low input impedance may be employed. A specific embodiment of a current detector as illustrated includes, by way of example, a PNP transistor of a type intended for medium-speed general-purpose applications and similar to the 2N2l9 transistor manufactured by the Radio Corporation of America. The two diodes D and D are made of germaniurn and silicon, respectively. The bias resistors 43 and 44 are 6.8K and 1.8K ohms, respectively, and the coupling resistors, such as the resistor ii, are all selected to be 39K ohms.

While the principles of the invention have not been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications in structure, arrangement, proportions, the

elements, materials, and components, used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from those principles. For instance, as noted hereiubefore, the digital switches illustrated are relays actuated by electrical signals derive from devices being monitored, but any switch actuated by a mechanical analog of the electrical signals applied to the relay-type digital switches may be employed. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What is claimed is:

l. A scanning device comprising: a plurality of input terminals; a plurality of series circuits one for each input terminal, each series circuit having two terminals and comprising a current switch and an impedance element, one terminal being connected to a respective one of said input terminals; a source of reference potential, at current detecting means having an input terminal connected to the second terminal of each of said series circuits and an output terminal for producing an output signa in response to an input current signal through any one of said impedance elements, current detecting means having a low input impedance relative to the impedance of a given one of said impedance elements, whereby a current signal through any one of said in.- pedance elements is substantially short-circuited to said source of reference potential; a source of potential; means for selectively connecting said source of potential 0 one of said input terminals; and a plurality of actuating means coupled to said current switches, one actuating means for each current switch for rendering it conductive.

2. A scanning device com ng: a plurality of input terminals; a plurzdity of series circuits, one for each input terminal, each series circuit having two terminals and including a current switch and an impedance element, one terminal being connected to a respective one of said input terminals; aicommorremitteuconnected transistor amplifier including a transistor connected in a common-emitter amplifier configuration, its base electrode being connected to the second terminal of each said series circuits and its collector electrode being coupled to an output terminal for producing an output signal in response to an input current signal through any one of said impedance elements connected to said base electrode, said commoncmitter-connected transistor amplifier having a low input impedance relative to the impedance of a given one or" said impedance elements, whereby a current signal through any one of said impedance elements is substantially shor-circuited to said source of emitter bias potential; a source'of potential of a given polarity selected to render the said transistor conductive when applied to its base through one of said impedance elements; means for selectively connecting said source of potential to one of said input terminals; and a plurality of actuating means coupled to said current switches, one actuating means for each current switch for rendering it conductive.

3. In a system for selectively transferring groups of digital signals to a common utilization device, wherein said digital signals are manifested by the conductive condition of current switches, the combination comprising: a plurality of current switches arranged in equal groups, each current switch having an input and an output terminal; a plurality of bus bars, one for each group or" current switches connected to the input terminal of the current switches thereof; a plurality of impedance elements, one for each current switch, each impedance element having a first terminal connected to the output terminal of its associated current switch and a second terminal; a group of current detectors, one for each current switch of a given group of current switches, each having an input terminal, an output terminal, and low input impedance rclati 'e to the impedance of a given one of said impedance elements; means for connecting the second terminal of the impedance elements connected to a different one 05 said current switcl s in each group to a corresponding one of said current detectors; a source of potential; and means for selectively connecting said source of potential to one of said bus bars, whereby a group of digital signals manifested by the conductive condition of a group or" current switches connected to the selected one of said bus bars and coupled to corresponding ones of said current detectors by associated impedance elements is translated into a group of voltage signals at output terminals of said current detectors and a digital signal translated into a current signal at an input terminal of a given current detector is substantially short-circuited to a source of reference potential.

4. in a system for selectively transferring groups of digital signals to a common utilization device, wherein said digital signals are manifested by the conductive con-- dition of current switches, the combination comprising: a plurality of current switches arranged in equal groups of at least two switches, each on rent switch having an input and an output terminal; a plurality of bus bars, one associated with each group of current S'\ 'itches, each one being connected to the input terminal of the current switches associated therewith; a plurality of impedance elements, one for ea h current switch, each impedance element having a first terminal connected to the output terminal of its associated current switch and a second terminal; a plurality of transistor amplifiers, one for each current switch of a given group of current switches, each amplifier including a transistor connected in a commoncmittcr amplifier configuration having an input terminal connected to the base electrode, an output terminal connected to the collector electrode, and a low input impedance throu h the base and emitter circuits relative to the impedance of a given one of said impedance elements; means for connecting the second terminal of the impedance elements connected to a different one of said current switches in each group to the input terminal of a corresponding one of said common-emitter transi tor amplifiers; a source of potential of a given polarity selected to render a given one of said transistor amplificrs conducted when applied to its input terminal through one of said impedance elements; a means for selectively connecting said source of potential to one of said bus bars whereby a group of digital signals manifested by the conductive condition of a group of current switches connected to the selected one of said bus bars and coupled to said common-emitter transistor amplifiers by associated impedance elements is translated into a group of voltage signals at output terminals thereof and a digital signal translated into a current signal at an input terminal of a given common-emitter transistor amplifier is substantially sh0rt-circuited through the base and emitter circuits thereof.

5. In a system for selectively transferring groups of 11 digital signals to a common utilization device, wherein said iZ digital signals are manifested by the conductive condition of a selected group of 1 current switches, the combination comprising a group of n common-emitter transistor amplifiers each havin an output terminal adapted to be connected to said utilization device and an input terminal; a plurality of groups of 11 current switches, each current switch of a given group having an output terminal coupled to a different one of said amplifiers and an input terminal; a plurality of actuating means, one actuating means coupled to each current switch for rendering it conductive; a plurality of bus bars, one for each group of current switches connected to the input terminal of the current switches thereof; a source of potential of a given polarity selected to render a given one of said transistor amplifiers conductive when applied to its input terminal through one of said impedance elements;

and means for selectively connecting said source of potion of a group of 12 current switches connected to the selected one of said bus bars and coupled to corresponding ones of said transistor amplifiers by associated impedance elements is translated into a group of voltage signals at output terminals of said transistor amplifiers and a current signal translated to an input terminal of a given transistor amplifier is substantially short-circuited by the base and emitter circuits thereof.

6. In a system for selectively scanning a plurality of bidirectional-conductive switches and transferring in pmallel a selected group of digital signals manifested by the conductive condition of associated switches to a common utilization device, the combination comprising: a plurality of switches arranged in equal groups, each switch having an input terminal, an output terminal, and a control element; a plurality of actuating means, one actuating means connected to the control element of each switch for rendering it conductive between its input and outnut terminals; a plurality of bus bars, one for each group of switches connected to the input terminal of the rality of impedance elements, one for each switch, each impedance element being serially connected between the output terminal of its associated switch and the input terminal of a corresponding one of said amplifiers; a source of potential of a given polarity selected to render a given one of said amplifiers conductive when applied to its input terminal through one of said impedance elements; and means for selectively connecting said source of potential to one of said bus bars whereby a selected group of digital signals manifested by the conductive condition of a group of switches connected to the selected one of said bus bars is translated into a group of voltage signals at output terminals of said amplifiers a current signal at an input terminal of a given amplifier is substantially short-circuited through the base and emitter circuits thereof.

References Cite by the Examiner UNITED STATES PATENTS 2,386,482 10/45 Leathers et al. 340-147 2,627,039 1/53 Mac'Williarns 340l47 2,628,277 2/53 Spencer 340-447 FOREIGN PATENTS 1,238,923 7/60 France.

NEIL C. REA-D, Primary Examiner. 

1. A SCANNING DEVICE COMPRISING: A PLURALITY OF INPUT TERMINALS; A PLURALITY OF SERIES CIRCUITS ONE FOR EACH INPUT TERMINAL, EACH SERIES CIRCUIT HAVING TWO TERMINALS AND COMPRISING A CURRENT SWITCH AND AN IMPEDANCE ELEMENT, ONE TERMINALS BEING CONNECTED TO A RESPECTIVE ONE OF SAID INPUT TERMINALS; A SOURCE OF REFERENCE POTENTIAL, A CURRENT DETECTING MEANS HAVING AN INPUT TERMINAL CONNECTED TO THE SECOND TERMINAL OF EACH OF SAID SERIES CIRCUITS AND AN OUTPUT TERMINAL FOR PRODUCING AN OUTPUT SIGNAL IN RESPONSE TO AN INPUT CURRENT SIGNAL THROUGH ANY ONE OF SAID IMPEDANCE ELEMENTS, SAID CURRENT DETECTING MEANS HAVING A LOW INPUT IMPEDANCE RELATIVE TO THE IMPEDANCE OF A GIVEN ONE OF SAID IMPEDANCE ELEMENTS, WHEREBY A CURRENT SIGNAL THROUGH ANY ONE OF SAID IM- 